AP Biology

## Learning Outcomes

- Define shared derived characters and distinguish them from ancestral characters.
- Explain how shared derived characters provide evidence for common ancestry.
- Use a simple character matrix to infer relationships among taxa.
- Interpret nodes, branches, sister taxa, and clades on phylogenetic trees and cladograms.
- Distinguish homologous traits from analogous traits when evaluating phylogenetic evidence.
- Apply parsimony to choose the tree that requires the fewest evolutionary changes.
- Avoid common AP Biology mistakes when explaining evolution and phylogeny.

## AP Biology Syllabus

In AP Biology Unit 7, phylogeny connects evolution, common ancestry, and evidence from morphology, DNA, RNA, proteins, and development. Shared derived characters are especially useful because they help biologists infer which organisms share a more recent common ancestor. Exam questions may ask you to interpret a cladogram, identify a shared derived trait, justify a relationship using data, or explain why a trait is not good evidence for close ancestry.

- Phylogenetic trees and cladograms represent hypotheses about evolutionary relationships.
- Shared derived characters are used to identify clades and recent common ancestry.
- Traits may be anatomical, molecular, developmental, behavioral, or physiological.
- Homologous traits support common ancestry; analogous traits can mislead phylogenetic analysis.
- Derived characters can be gained, modified, or lost over evolutionary time.
- DNA and amino acid sequence comparisons can be used as character evidence.
- Exam responses should explain that populations evolve over generations, not individuals.
- Claims about relatedness should be justified with specific traits or sequence data.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. What is the difference between an ancestral character and a derived character?
2. Why is hair a useful shared derived character for identifying mammals, but a backbone is not useful for separating mammals from other vertebrates?
3. In a cladogram, what does a node represent?
4. Why can wings in bats and insects be poor evidence that bats and insects are closely related?
5. How can DNA sequence similarities be used to support or revise a phylogenetic tree?

## Introduction

Phylogeny is the study of evolutionary relationships among organisms. In AP Biology, you are not expected to memorize every branch of the tree of life, but you are expected to interpret diagrams and data that show how organisms may be related through common ancestry. A phylogenetic tree or cladogram is a model: it represents a scientific hypothesis based on evidence.

Shared derived characters are one of the main kinds of evidence used to build those models. A character is any heritable feature that can be compared among organisms, such as a backbone, amniotic egg, type of flower, DNA sequence, or protein sequence. A derived character is a version of a trait that evolved after a lineage split from other lineages. When multiple taxa share that derived character, it can indicate that they inherited it from a recent common ancestor.

> **Key Term: phylogeny**
> The evolutionary history and relationships among organisms or groups of organisms.
>
> **Key Term: shared derived character**
> A trait that evolved in a recent common ancestor and is shared by its descendants.

### How shared derived characters reveal common ancestry

To understand shared derived characters, first separate two ideas: ancestral characters and derived characters.

An ancestral character is a trait that was already present in an older common ancestor. Because many groups may inherit it, it may not help distinguish which groups are most closely related. A derived character is a newer form of a trait that appeared after a branch split from other lineages. If several organisms share that derived trait, it suggests that they inherited it from a common ancestor that had the trait.

For example, consider vertebrates:

- A vertebral column is shared by fishes, amphibians, reptiles, birds, and mammals.
- Jaws are shared by many vertebrates but not by jawless fishes.
- Four limbs are shared by amphibians, reptiles, birds, and mammals.
- An amniotic egg is shared by reptiles, birds, and mammals.
- Hair is shared by mammals.

If you are trying to identify mammals as a group, hair is more useful than a vertebral column. A vertebral column is ancestral for mammals because it appeared earlier in vertebrate evolution and is shared with many non-mammalian vertebrates. Hair is a shared derived character for mammals because it evolved in the mammalian lineage and is found in mammals.

> **Key Term: ancestral character**
> A trait inherited from a distant ancestor and shared with multiple lineages, so it may not identify recent common ancestry.
>
> **Key Term: derived character**
> A trait that evolved after a lineage split from other lineages.

A shared derived character is also called a synapomorphy. AP Biology questions may use either phrase. For example, if all members of a group have a certain DNA insertion that related outside groups lack, that insertion may be treated as a shared derived character for the group.

> **Key Term: synapomorphy**
> Another name for a shared derived character used to identify a clade.

The key exam idea is that shared derived characters are evidence for common ancestry because they are unlikely to have appeared in exactly the same inherited form in several related lineages by chance. This is especially true for complex molecular changes, such as a specific DNA sequence change shared by multiple taxa.

> **Test Tip:**
> When asked why two organisms are closely related, do not simply say they “look alike.” Identify a specific shared derived character or shared DNA/protein similarity and connect it to inheritance from a recent common ancestor.

Shared derived characters can be physical or molecular. Anatomical examples include feathers in birds, flowers in angiosperms, and mammary glands in mammals. Molecular examples include DNA base substitutions, insertions, deletions, gene duplications, or amino acid sequence similarities. Molecular data are often useful because all organisms use DNA or RNA as genetic information, allowing comparisons even among organisms with very different body plans.

A key point from gene expression and molecular biology is that different nucleotide sequences can sometimes code for the same amino acid because the genetic code is redundant. This means two species may have different DNA sequences but identical protein sequences. For phylogeny, both DNA and amino acid data can be useful, but they do not always show the same amount of variation.

### Reading cladograms using nodes, branches, and clades

A cladogram is a branching diagram that shows hypothesized relationships based on shared derived characters. It does not necessarily show exact time, amount of genetic change, or direct ancestor-descendant relationships unless the diagram specifically includes that information.

> **Key Term: cladogram**
> A branching diagram that shows hypothesized evolutionary relationships among taxa based on shared characteristics.
>
> **Key Term: taxon**
> A named group of organisms at any level, such as a species, genus, family, or larger group.

On a cladogram, the tips of the branches usually represent taxa being compared. These could be species, genera, or larger groups. Branch points, or nodes, represent common ancestors. A node shows where one ancestral lineage split into two or more descendant lineages.

> **Key Term: node**
> A branch point on a phylogenetic tree or cladogram that represents a common ancestor.

A clade includes an ancestor and all of its descendants. For example, mammals form a clade if the group includes the common ancestor of all mammals and every descendant of that ancestor. A true clade is monophyletic.

> **Key Term: clade**
> A group consisting of a common ancestor and all of its descendants.
>
> **Key Term: monophyletic group**
> A group that includes a common ancestor and all descendants of that ancestor.

Two taxa that share an immediate common ancestor are called sister taxa. Sister taxa are more closely related to each other than either is to taxa outside their shared node.

> **Key Term: sister taxa**
> Two lineages that share an immediate common ancestor.

A common AP skill is reading the order of branching correctly. The closer the shared node, the more recent the common ancestor. However, do not judge relatedness by the order of taxa across the page. Branches can rotate around nodes without changing the relationships. A tree drawn with humans at the far right does not mean humans are “most evolved” or that evolution has a ladder-like direction.

> **Exam Warning:**
> Do not say one modern species “evolved from” another modern species on a cladogram. Modern taxa usually share common ancestors; one living species is not shown as the direct ancestor of another unless the diagram clearly states that.

For example, if a cladogram shows frogs branching off before lizards and mice split from each other, then lizards and mice share a more recent common ancestor with each other than either shares with frogs. This does not mean frogs are primitive or unchanged. Frogs, lizards, and mice have each been evolving since their lineages separated.

This links back to a major natural selection idea: populations evolve over generations. Natural selection acts on individuals with different phenotypes, but evolutionary change is measured in populations over time. Phylogenetic diagrams summarize the results of many such changes across lineages.

### Building relationships from character evidence

A simplified character analysis asks: Which taxa share which derived characters? Biologists may begin with an outgroup, a taxon known or hypothesized to be less closely related to the other taxa. The outgroup helps identify which traits are ancestral and which are derived.

> **Key Term: outgroup**
> A taxon outside the main group being studied, used as a reference for identifying ancestral and derived traits.
>
> **Key Term: ingroup**
> The taxa being studied for their evolutionary relationships.

Suppose you are comparing five vertebrate taxa: lamprey, perch, frog, lizard, and mouse. You examine these characters:

- Vertebral column
- Jaws
- Four limbs
- Amniotic egg
- Hair

Lamprey has a vertebral column but lacks jaws, four limbs, an amniotic egg, and hair. Perch has jaws but lacks four limbs, an amniotic egg, and hair. Frog has four limbs but lacks an amniotic egg and hair. Lizard has an amniotic egg but lacks hair. Mouse has hair.

From these characters, you could infer a branching pattern like this:

- The vertebral column appears before all listed taxa.
- Jaws appear after the lamprey lineage branches away.
- Four limbs appear after the perch lineage branches away.
- The amniotic egg appears after the frog lineage branches away.
- Hair appears on the mouse lineage.

In this example, the amniotic egg is a shared derived character for lizards and mice relative to frogs, perch, and lamprey. Hair is a derived character for mice, but it is not shared among the listed taxa unless more mammals are included.

> **Key Term: character matrix**
> A data set showing the presence, absence, or state of traits across the taxa being compared.

A character matrix often uses 0 and 1 to code trait states. For example, 0 might mean a trait is absent and 1 might mean it is present. AP questions may give you a matrix and ask which tree is best supported by the data. Focus on shared derived characters, not just total number of shared traits.

Consider this prompt:

> **Question:**
> Species A, B, C, and D are compared. Species A is the outgroup and lacks traits 1, 2, and 3. Species B has trait 1 only. Species C has traits 1 and 2. Species D has traits 1, 2, and 3. Which species are most closely related?
>
> **Answer:**
> Species C and D are most closely related because they share trait 2, a derived character not found in species A or B. Species D also has trait 3, but that trait evolved after the C-D common ancestor split into separate lineages.

This kind of reasoning is more valuable on the exam than memorizing a named tree. You must use evidence to support a claim.

Another principle often used in tree building is parsimony. Parsimony favors the hypothesis that requires the fewest evolutionary changes, unless other evidence supports a more change-heavy explanation.

> **Key Term: parsimony**
> The principle that the simplest phylogenetic explanation requiring the fewest evolutionary changes is preferred when evidence is otherwise equal.

For example, if three species share a rare DNA deletion at the same location in a gene, the most parsimonious explanation is usually that the deletion happened once in their common ancestor, rather than independently three separate times. Parsimony does not prove a tree is correct; it helps choose among competing hypotheses.

> **Test Tip:**
> In a free-response answer, make the claim first, then cite the shared derived character, then explain inheritance from a recent common ancestor. For example: “Species X and Y are sister taxa because they both have trait 4, which is absent in the outgroup and other taxa, indicating inheritance from their most recent common ancestor.”

### Homology, misleading similarities, and molecular data

Not every shared trait is a shared derived character. Some similarities exist because organisms inherited a trait from a distant ancestor. Other similarities arise because unrelated organisms face similar environmental pressures and evolve similar adaptations independently.

A homologous character is similar because of common ancestry. The forelimbs of humans, cats, whales, and bats have similar bone arrangements because they were inherited from a common tetrapod ancestor, even though they now serve different functions such as grasping, walking, swimming, or flying.

> **Key Term: homologous character**
> A trait shared because it was inherited from a common ancestor.

An analogous character is similar in function or appearance but did not arise from a recent common ancestor with that trait. Wings in insects and birds both function in flight, but they are structurally and evolutionarily different. Bird wings are modified vertebrate forelimbs with bones; insect wings are extensions of the exoskeleton. If you used “has wings” alone to group birds with insects, you would infer a misleading relationship.

> **Key Term: analogous character**
> A similar trait that evolved independently in different lineages rather than being inherited from a recent common ancestor.

This distinction matters because natural selection can favor similar phenotypes in similar environments. The environment determines which phenotypes are favorable; individuals with advantageous phenotypes have higher reproductive success, and over generations those phenotypes may become more common. Similar selection pressures can produce similar-looking traits in distantly related organisms.

> **Exam Warning:**
> Do not treat every similarity as evidence of close relatedness. Ask whether the similarity is homologous and derived, or whether it may be analogous or ancestral.

Molecular evidence can reduce some of the uncertainty caused by outward appearance. DNA, RNA, and amino acid sequences provide many characters that can be compared. Closely related species usually have more similar sequences because less time has passed since their common ancestor. More differences often suggest a more distant common ancestor, although mutation rates can vary among genes and lineages.

For example, suppose three species are compared using a short DNA sequence:

- Species 1: A T G C C A T
- Species 2: A T G C C G T
- Species 3: A C A T G G C

Species 1 and 2 differ at one position, while species 3 differs at several positions from both. This supports the hypothesis that species 1 and 2 share a more recent common ancestor with each other than either shares with species 3.

Protein comparisons can also support phylogeny. Because DNA codons are redundant, different DNA sequences may produce the same amino acid sequence. Therefore, amino acid sequences may show fewer differences than DNA sequences. If an exam asks why two species can have different nucleotide sequences but the same protein sequence, mention the redundancy of the genetic code.

Developmental traits can also help. For instance, similarities in early embryonic structures among vertebrates support common ancestry. However, AP questions usually require you to connect the evidence to a specific claim about relatedness rather than simply listing evidence types.

A final exam issue is trait loss. A derived character does not have to be a new visible structure. It may be the loss of a structure. For example, if a lineage loses functional eyes, that loss can be treated as a derived character if it is inherited by descendants. However, absence of a trait must be interpreted carefully. Two species may both lack a trait because neither ever had it, or because both lost it independently. Context from an outgroup and other characters is needed.

## Key Point Checklist

This article has covered the following key knowledge points:

- Phylogeny describes evolutionary relationships among organisms or groups.
- Shared derived characters are traits inherited from a recent common ancestor.
- Ancestral characters are older traits and may not identify close relationships.
- A cladogram is a hypothesis based on evidence, not a fixed fact or ladder of progress.
- Nodes represent common ancestors where lineages split.
- A clade includes a common ancestor and all of its descendants.
- Sister taxa share an immediate common ancestor.
- Outgroups help identify which traits are ancestral and which are derived.
- Character matrices can be used to infer branching patterns from trait data.
- Homologous characters support common ancestry; analogous characters can mislead analysis.
- Molecular data, including DNA and amino acid sequences, can provide evidence for phylogeny.
- Parsimony favors the tree requiring the fewest evolutionary changes when evidence is otherwise equal.

## Key Terms and Concepts

- phylogeny
- shared derived character
- ancestral character
- derived character
- synapomorphy
- cladogram
- taxon
- node
- clade
- monophyletic group
- sister taxa
- outgroup
- ingroup
- character matrix
- parsimony
- homologous character
- analogous character


Phylogeny - Shared derived characters